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Full text of "Text Book Of Mechanical Engineering"

Appendix: TIL

927

It appears, then, that an electrical H.P. (E.H.P.) is 746
watts, or 33,000 foot pounds per minute, as before; and the only
reason for the name 'electrical' is to indicate that it is power given
off a dynamo or to a motor. A kilowatt consists of 1000 watts.

The Board of Trade unit for quantity of energy supplied is
Jooo watts acting for i hour, or 1000 ivatt-hours.

Board of Trade
Electrical unit

746

ii| H.P. for i hour.

Efficiencies. — The losses in a dynamo are said to consist of r/wo
parts, the one due to electric resistance of the wire coils, and the
other to friction, hysteresis, and eddies, called stray energy.
Now, if we could imagine a dynamo whose only loss was that of
current resistance, or C2R-=-746 (see p. 553), E.H.P. being that
given out}

ElectricaUfficiency  =

Similarly we may suppose the only loss to be the stray
energy, and the efficiency could then be reckoned on that alone.
Take H.P. to be that put in, we might say

Mechanical efficiency

H.P. —stray power
H~.R

But, as in all dynamos, there are both electrical and mechanical
losses, the nett efficiency, or

Commercial efficiency = mech. effy. x elec. eify.

and thus we account for all losses. The H.P. supplied is often
spoken of as mechanical H.P., while that gwen off is electrical
H.P. Hysteresis is a heat loss caused by resistance to magnetisa-
tion and demagnetisation.

Example 67.—A dynamo is designed to produce 200 amp&res of
current at a pressure of 100 volts. State this in Board of trade units.
Calculate also the rnedhaaical H.P. required to drive the machine,
and the E.H.P. given off, if the commercial efficiency is 85 °/0,